1. Technical Field
The present invention relates to a wire-bonding method of connecting a first bonding point with a second bonding point by a wire, and to a semiconductor device having a wire-loop shape formed by connecting a first bonding point with a second bonding point by a wire.
2. Description of the Related Art
For assembling a semiconductor device, wire bonding for connecting a pad of a semiconductor chip mounted on a lead frame with a lead of the lead frame by a thin metal wire is used. A wire-bonding apparatus is used in the wire bonding, in which an initial ball is first formed at a tip end of the wire, and the initial ball is pressure-bonded to the pad of the semiconductor chip using the capillary, thereby forming a pressure-bonded ball. In this method, after moving the capillary upward to make a reverse motion in a direction away from a second bonding point, the capillary is further moved upward to a predetermined height, and then moved toward the second bonding point, thereby the wire is connected to the second bonding point (see FIG. 4 to FIG. 6 of Japanese Unexamined Patent Application Publication No. 2004-172477 (hereinafter referred to as “Patent Document 1”), for example).
The wire is bonded by moving the capillary in this manner, and, in many cases, a shape of a wire loop is formed either in a triangular shape including a wire neck that extends upward from the pressure-bonded ball that has been pressure-bonded onto the pad of the semiconductor chip and a sloped portion that has been bent toward the second bonding point at the wire neck, or in a trapezoidal shape including a flat portion that extends substantially horizontally in a direction of the second bonding point from the wire neck and a sloped portion that extends from the flat portion toward the second bonding point. This is because, when a portion close to the pressure-bonded ball is moved horizontally in the direction of the second bonding point with respect to the capillary, the neck portion is often damaged due to a friction between the capillary and the thin metal wire that is produced while moving.
However, the height of the wire loop in this wire-loop shape is high because the wire neck that rises from the pressure-bonded ball is included, and this poses a problem that the height or thickness of the semiconductor device assembled by wire bonding as a whole cannot be made small.
Therefore, a method has been proposed, making a reverse motion such that a capillary is moved slightly upward and away from a second bonding point after bonding to a first bonding point, making a forward motion such that the capillary is further moved slightly upward and in a direction toward the second bonding point, then moving the capillary downward to press a wire neck portion against a pressure-bonded ball and fold the wire neck portion up on the pressure-bonded, making the wire to extend either in a horizontal direction or in a direction slightly sloped upward from the horizontal direction, moving the capillary upward while feeding the wire from a tip end of the capillary, and then moving the capillary toward the second bonding point, thereby connecting the wire to the second bonding point (see FIG. 1 to FIG. 3 of Patent Document 1, or FIG. 1 to FIG. 3 of Japanese Unexamined Patent Application Publication No. H09-51011 (hereinafter referred to as “Patent Document 2”), for example).
According to the conventional bonding method described in the Patent Document 1 or 2, as a wire is folded up onto a pressure-bonded ball and then pressed to form a head portion, the head portion cannot be made particularly low. As a result, the conventional bonding method occasionally fails to satisfy the demand for decreasing the height of the wire loop as a whole.